HUMAN BIOLOGY

How many genes for a trait does a person

inHerit?

• Each of us inherits two copies of each gene, one on each
  member of a pair of homologous chromosomes.


• If the two inherited alleles of a gene are identical, the person
  is homozygous for the trait. If the two inherited alleles are
  different, the person is heterozygous for the trait.

taKe-Home messaGe

allele Term for each chemi-

cal version of a gene.

gene A unit of DNA that

provides the chemical

instructions for building a

protein.

genotype All the alleles a

person inherits.

heterozygous Having two

different alleles for a trait.

homozygous Having the

same two alleles for a trait.

locus The location of a

gene on a chromosome.

phenotype Observable

genetic traits.

n Genes provide the instructions for all human traits,

including physical features and how body parts function.

Each person inherits a particular mix of maternal and

paternal genes.

n Link to Reproduction and the chromosome number 18.1

Genes are units of DNA that provide chemical instruc-

tions for building proteins. The proteins in turn provide

information necessary to produce

specific traits. Each gene has a spe-

cific location, called a locus, on a

given chromosome. In all, humans

have about 21,500 genes, which pass

from parents to offspring by way of

the parental chromosomes that are

united in the nucleus of a fertilized

egg. All diploid body cells have two

copies of each gene—one on each

member of each pair of homologous

chromosomes in the nucleus (Fig-

u r e 19.1).

All copies of a gene deal with the

same trait, but the genetic informa-

tion about it may vary somewhat

due to chemical differences between

copies. Each version of the gene is

called an allele. Some genes have two alleles; others, like

the gene for inherited freckles, have more. For a given trait,

however, a person may inherit only two, one from each

parent. The two alleles may be chemically identical or

chemically different. Differences between alleles produce

much of the variation we see in traits.

If a person inherits two

identical copies (alleles) of

a gene, then for that trait

the person is said to be

homozygous (homo-, “same”;

zygo, “joined together”). If the

two inherited copies are dif-

ferent, the person is said to

be heterozygous (hetero-,

“different”) for the trait.

An allele is dominant when

its effect on a trait masks that

of any recessive allele paired

with it. Dominant alleles are

represented by uppercase let-

ters, recessive ones by lower-

case (for instance, A and a, B

and b, and so on).

Someone who is homozy-

gous dominant for a trait has a

Basic Concepts of Heredity

pair of dominant alleles for the trait. Someone who is homo-

zygous recessive for the trait has a pair of recessive alleles.

A heterozygous person has a pair of nonidentical alleles.

The alleles a person inherits are his or her genotype

for the trait being considered. Observable functional

or physical traits are the phenotype (Figure 19.2 and

Ta ble 19.1).

Figure 19.1 Animated! Knowing a few basic genetic

terms will help you understand principles of inheritance.

A A pair of homologous chromosomes,

each in the unduplicated state (most

often, one from a male parent and its

partner from a female parent)

B A gene locus (plural, loci), the

location for a specific gene on

a specific type of chromosome

C A pair of alleles (each being

one chemical form of a gene)

at corresponding loci on a pair

of homologous chromosomes

D Three pairs of genes (at three

loci on this pair of homologous

chromosomes); same thing as

three pairs of alleles

19.1

Table 19.1 Freckling Genotypes and Phenotypes

Genotype Described as Phenotype

FF Homozygous dominant Freckles

Ff Heterozygous (one of Freckles

each allele; dominant

form of trait observed)

ff Homozygous recessive No freckles

© Cengage Learning

Figure 19.2 Many genetic traits,
including inherited freckling, are
governed by genes that have
dominant and recessive forms.
This mother and daughter both
must have at least one dominant
form of the freckling gene.

374 Chapter 19

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